NASA study reveals mysteries of neutron star

Ohio University astrophysicist Madappa Prakash is part of an international team of researchers that have discovered a remarkable state of matter in a high-density neutron star.

Neutron star Cassiopeia A, the remains of a supernova that occurred 330 years ago, has puzzled scientists by cooling at an unusually fast rate. Researchers now report that the quick cooling is the first direct evidence that the cores of neutron stars are made of superfluids and superconducting materials.This image presents a beautiful composite of X-rays from Chandra (red, green, and blue) and optical data from Hubble (gold) of Cassiopeia A, the remains of a massive star that exploded in a supernova. Evidence for a bizarre state of matter has been found in the dense core of the star left behind, a so-called neutron star, based on cooling observed over a decade of Chandra observations. The artist’s illustration in the inset shows a cutout of the interior of the neutron star where densities increase from the crust (orange) to the core (red) and finally to the region where the "superfluid" exists (inner red ball). Credit: X-ray: NASA/CXC/UNAM/Ioffe/D.Page,P.Shternin et al; Optical: NASA/STScI; Illustration: NASA/CXC/M.Weiss

Superfluidity is a friction-free state of matter, and superfluids created in laboratories on Earth exhibit remarkable properties, such as the ability to climb upward and escape airtight containers. Superfluids made of charged particles are also superconductors, which have widespread technological applications for producing superconducting magnets used for magnetic resonance imaging (MRI) machines.

Prakash was involved in one of two independent research teams that used data from NASA’s Chandra X-ray Observatory to study the dramatic cooling of Cassiopeia A. Neutron stars have intrigued scientists because they contain the densest known matter directly observable. One teaspoon of neutron star material weighs six billion tons. The pressure in the star’s core is high enough that most of the electrons there are forced to merge with protons, producing neutrons. This leaves a star composed mostly of neutrons, with some protons, electrons and other particles.

During Cassiopeia A’s cool-down phase, first proton pairs, and then neutron pairs, enter superconducting and superfluid phases, respectively, the scientists report. During this process, copious amounts of weakly-interacting particles known as neutrinos are emitted. The neutrinos rapidly escape from the star, taking energy with them. These neutrinos are responsible for the observed rapid cooling, according to the research team led by Dany Page of the National Autonomous University in Mexico. Page worked on the project with Prakash, James Lattimer of SUNY Stony Brook and Andrew Steiner of Michigan State University.

Madappa Prakash. Photo credit: Rick Fatica.

Finding a neutron star at this particular young age and at this stage of its life was crucial for the discovery, explained Prakash, an Ohio University professor of physics and astronomy.

“It turns out that Cas A may be a gift from the universe because we would have to catch a very young neutron star at just the right point in time,” he said. “Sometimes a little good fortune can go a long way in science.”

The new research not only pinpoints the temperature at which superfluidity occurs in neutron stars—between one half a billion to just under a billion degrees Celsius—but is important for understanding a range of behavior in neutron stars, including glitches, neutron star precession and pulsation, magnetar outbursts and the evolution of neutron star magnetic fields.

Astrophysicists should continue to monitor the star, which is expected to rapidly cool over the next few decades, and look for other examples of this activity in the cosmos, Prakash said.

“Can we find other young neutron stars that behave in the same way?” he asked. “Is this a universal phenomenon?”

With forthcoming journal publications from both international research teams this month, the new work on Cassiopeia A has captured the attention of the scientific community. The team including Prakash publishes its study this week in the journal Physical Review Letters. The second team, led by Peter Shternin of the Ioffe Institute in St. Petersburg, Russia, is slated for publication in the Monthly Notices of the Royal Astronomical Society. The team includes Dmitry Yakovlev of the Ioffe Institute; Daniel Patnaude of the Harvard-Smithsonian Center for Astrophysics; Craig Heinke of the University of Alberta, Canada; and Wynn Ho of the University of Southampton, UK. Heinke and Ho first discovered the Cassiopeia A cooling last year.